The present invention relates to a device for measuring the volume flow of a fluid in a pipe, comprising At least two objects with a different shape and/or diameter to be arranged in the pipe, a sensor for determining the frequency with which vortices occur which arise behind the objects upon the fluid flowing round the objects and means for determining the volume flow of the fluid from this frequency.
Such a device is known from DE-A-37 14 344.
In said document the vortex flow meter comprises two objects, or better said one object, which is splitted in two parts of different form. The purpose of this object (these two parts) is to provide for only one vortex frequency measurement. The two parts of the object must have substantially equal measures. Although in said document the occurrence of more vortex frequencies are described, the construction is directed to suppress the high frequency components by rounding off the corners of the object so that only one vortex frequency component results.
By placing an object in a fluid flow, a so-called Von Karman vortex street forms behind this object upon circumfluence of this object by the fluid. Such a vortex street is depicted in, for instance, Milton van Dyke, An Album of Fluid Motion (Stanford University, California, 1982. Depending on the shape and dimensions of the object, vortices arise in the fluid with a certain frequency. In general, this frequency fv can be represented by the following relation:       f    v    =            (                        S          r                D            )        *          U      0      
wherein Sr forms the Strouhal number determined by the shape of the object, D represents the diameter of the object, that is, the distance over which the fluid flow is interrupted by the object, and Uo represents the approach velocity of the fluid flowing towards the object. By measuring, with a known approach velocity, the frequency of the vortices, for every object the associated Strouhal number can be determined. Over a certain interval, the Strouhal number is found to be substantially independent of the Reynolds number Re and to have a constant value; this Reynolds number has the following relation with the above-mentioned approach velocity Uo:   Re  =            ρ      ⁢              xe2x80x83            ⁢              U        0            ⁢      d        η  
wherein xcfx81 represents the density of the fluid, d represents the diameter of the pipe through which the fluid is led and xcex7 represents the dynamic viscosity of the fluid. Accordingly, within the interval referred to, because the Strouhal number is independent of the fluid density and the viscosity of the fluid, there is a linear relation between the vortex frequency fv and the approach velocity Uo, as represented hereinbefore.
When the approach velocity is not constant, however, problems may arise in such a volume flow measuring device. Due to variations in the approach velocity presented, pulsation frequencies arise therein. The problems referred to here are dependent on the ratio between the pulsation frequency fp in the approach velocity presented and the vortex frequency fv. From the article by M. C. A. M. Peters et al., Impact of pulsations on vortex flowmeters, Paper presented at FLOMEKO""98, Lund, Sweden Jun. 15-17, 1998, it appears that when fv/fp less than 0.4 and fv/fp greater than 2.5, the vortex frequency fv is found to take a value that corresponds to a vortex frequency associated with an approximately average approach velocity. In both cases, there is a unique linear relation between vortex frequency and approach velocity, and the latter quantity can be determined by measurement of the vortex frequency. When, by contrast, it holds that 0.4 less than fv/fp less than 2.5, so-called lock-in phenomena occur. xe2x80x9cLock-inxe2x80x9d means that within defined limits, with variations in the approach velocity, the vortex frequency remains the same, that is, the vortex frequency within these limits is strongly dominated by the pulsation frequency in the approach velocity. From the article referred to, in particular FIGS. 11 and 12, it appears that these vortex frequencies dominated by the pulsation frequency occur at fv/fp ratios of xc2xd, 1, 1xc2xd and 2. At amplitudes in the pulsation frequency of about 5% of the approach velocity, the errors in the measured vortex frequency are found to lie between +8% and xe2x88x9218%. Such errors lead to equal errors in the approach velocity to be determined. Accordingly, at a pulsating approach velocity with 0.4 less than fv/fp less than 2.5, the known approach velocity measuring devices of the type set forth in the preamble are highly unreliable.
The object of the invention is to obviate this disadvantage and to provide a device of the type described in the preamble which, also for random pulsations, still enables an accurate determination of this approach velocity.
To that end, according to the invention, the device such as it is described in the preamble is characterized in that at least two objects of so different a shape and/or diameter are present that at least one of the vortex frequencies thereby determined is independent of a pulsation frequency possibly arising in the approach velocity of the fluid.
By placing two objects of a different shape and/or diameter in the pipe, the Sr/D ratio for the two objects can be chosen to be so different that for a given pulsating approach velocity at least one of the two vortex frequencies has a value such that the fv/fp ratio comes to lie outside the interval (0.4, 2.5). Only from such a vortex frequency can a correct approach velocity be determined. Obviously, this requires the presence of a sensor to determine the pulsation frequency. In many cases, for both vortex frequencies the fv/fp ratio will lie outside this interval; in such a situation, by measurement of the vortex frequency in each of the two vortex streets, the same value for the approach velocity will be obtained. Although, of course, more than two objects may be placed in the pipe, this is basically unnecessary, and in practice only two objects will suffice.
The two objects referred to can be arranged after each other, while choosing the mutual distance to be sufficiently great to keep any mutual interaction of the two objects as small as possible. Such interaction can also be minimized by arranging the two objects crosswise in the pipe. The objects can be arranged not only behind each other, but also next to each other, again subject to the requirement that their mutual distance be sufficiently great to reduce interaction between the objects to a minimum. In addition, it is also possible, certainly when the diameter of the pipe is great relative to the dimensions of the objects, to design the two objects as one whole.
Both the measurement of the vortex frequency and the measurement of the pulsation frequency are done by means of pressure sensors which are arranged at a suitable point on or in the wall of the pipe, although it is also possible for the sensors determining the vortex frequency to be arranged in or on the respective objects, since the vortices induce a force acting on the objects that can be measured. Further, it is possible to integrate the sensor for determining the pulsation frequency into one of the sensors or into both sensors for determining the vortex frequency.